For most of the past few million years, our evolutionary ancestors' survival depended on being able to consume as much as possible when food was available and to store the excess energy for periods when it was scarce. In the developed world today, high-calorie foods are readily obtainable, yet the propensity to store fat is part of our species' heritage, leaving an increasing number of the world's people vulnerable to obesity. In an environment of abundant food, we are anatomically, physiologically, metabolically, and behaviorally programmed in a way that makes it difficult for us to avoid gaining weight.